I’m definitely not an expert but a long while back I did some playing around with the extension with the primary purpose to figure out if one could offload the LSP work to a separate openHAB instance from the production openHAB. The reason was at that time (perhaps still?) the LSP can cause a significant load on the openHAB instance and for some users it was adding lots of latency while they were editing files.
And that makes some sense when one realizes the LSP does the same thing that happens when loading a .rules file. So if there is a heavy hit to the CPU when loading the .rules file, there should be a heavy hit when editing that file and as you type the syntax is checked by the LSP.
If I recall correctly it worked pretty well for the syntax checking. However, it will only be aware of the Things, Items, and Sitemaps that exist on the openHAB instance the extension is connected to. So if you have two or more instances of openHAB, it’ll only know about the Items and such on the one the extension is connected to for the LSP.
Keep in mind this is all pretty old information. I suspect the extension has changed a bit since then. But I post it here in case it’s informative or useful.